1. Field of the Invention
The present invention relates to strike baths containing zinc and copper, and designed to provide improved adhesion to zinc and other metal substrates of strike coatings electrodeposited from such baths on such substrates. The resulting improved adhesion of the strike coating also improves the adhesion to such substrates of plating coatings electrodeposited over said strike coating.
2. Description of the Prior Art
Zinc based die-castings have been in use for many years for fabricating parts for automotive use, household appliances, and many other parts that are advantageously fabricated from zinc based die-castings.
The method of preparing zinc based die-castings for subsequent electroplating is well-known in the art and is fully discussed in standard textbooks. The coating process cycle generally involves pre-coating operations designed to clean the metal substrate to be electroplated. These cleaning operations can include vapor degreasing, emulsion cleaning, cleaning in di-phase type cleaners and/or cleaning with various detergents. The primary purpose of the pre-coating cleaning step is to remove the bulk of any soil that may be contaminating the surface of the metal substrate to be electrocoated. The next step is an aqueous rinsing of the cleaning agents followed by an alkaline cleaning using an anodic or cathodic electrocleaning process, with the former process being preferred. After another aqueous rinsing to remove the alkaline cleaning agents, the castings are neutralized in a very mild acid dip, followed by a thorough aqueous rinsing. The next step in the prior art cycle is copper striking in a cyanide copper strike bath to promote adhesion of the composite electrodeposit to the die-casting substrate. After the castings have been initially coated with a thin layer (about 5 to 50 microinches) of copper from this copper strike solution, they are generally plated to a heavy thickness (about 0.1 to 0.5 mils) from a cyanide copper plating bath or any other suitable copper plating bath. The thus coated die-casting can then be finished by plating with nickel followed by chromium or plated with silver, gold, or any other desired finish.
Typical methods for preparing zinc base die-castings for plating are fully described in ASTM, B252-69(77) and summarized in Metal Finishing Guidebook 1984, page 173. All of the treating solutions used for the preparation of zinc die-castings for plating are important in order to achieve the required adhesion of the final electrodeposit to the zinc substrate. However, the most critical solution is the copper strike electrolyte.
The major difference between a copper strike electrolyte or solution and a copper plating solution, is the concentration of copper used in the respective solutions. Cyanide copper strike solutions contain about 15 to 30 g/l (grams/liter) of copper metal and about 5 to 15 g/l of free cyanide. Cyanide copper plating solutions used for electroplating heavy coppers layers contain about 60 to 75 g/l of copper metal and about 7 to 15 g/l of free cyanide. The lower metal content of the copper strike solutions promotes good adhesion of the composite electrodeposit to the zinc substrate.
The term composite electrodeposit means, in accordance with the present invention, the combination of the relatively thin strike coating and the subsequently applied relatively thick plating coating. The strike coating is electrodeposited and the subsequent plating coatings may be electrodeposited or electroless coatings. Only one strike coating is usually used, whereas one or more plating coatings may be applied over the strike coating. Where a plurality of the plating coatings are used, they may be based on the same or different metals as listed above.
The use of cyanide compounds in commercial electroplating baths is not desirable due to the extremely poisonous nature of such materials. The effluent from these cyanide-based baths must be treated chemically to remove all cyanides before being discharged to an environmentally acceptable waste effluent. The industry has been searching for many years for a cyanide-free plating sequence for plating zinc, and in particular zinc based die-castings, which is also capable of imparting the required degree of adhesion needed for such materials and thus equal to the adhesion that can now be achieved by using cyanide-based solutions.
Alkaline cyanide-free copper plating baths are known in the art, and comprise copper metal and a suitable amount of a complexing or chelating agent. These baths contain about 15 to 35 g/l of copper metal and a (stoichiometric) excess of complexing or chelating agent above that required to keep the copper in solution. Many chelating and complexing agents have been used both in laboratories and in industry for these baths. They include phosphate type compounds such as pyrophosphates, organic phosphonates, complex phosphates; carboxylate compounds such as citrates, bluconates, glucoheptonates; amine carboxylate compounds such as ethylenediaminetetracetic acid (EDTA), nitrilotriacetic acid (NTA) or glycine or its derivatives; and organic amines. Examples of organic phosphonate copper plating baths are found in U.S. Pat. Nos. 3,833,486 and 3,475,293. Examples of copper-zinc alloy plating baths based on glucoheptonates are found in U.S. Pat. Nos. 4,356,067, 4,389,286 and 4,417,956.
Examples of copper strike baths for plating zinc die-castings is given in Plating, Vol. 55, Mar. 1968, p. 233-7. Alkaline copper strike baths are described based on pyrophosphates and good adhesion is claimed when the bath is ultrasonically agitated. Without ultrasonic agitation, adhesion is poor. This method for plating zinc die-castings was not accepted by industry due to the high cost of the relatively large ultrasonic equipment installations needed for such purposes.
Although cyanide-free alkaline copper plating baths are known, none of these have achieved wide industry acceptance as suitable for plating directly onto zinc, and in particular zinc based die-castings, since they cannot achieve a satisfactory degree of adhesion, such as that equal to the degree of adhesion that can be achieved from cyanide-based baths. The prior art copper strike baths that are low in copper concentration and are based on cyanide-free formulas also results in an unsatisfactory level of adhesion that is not equal to that obtained when using cyanide copper strikes.
The poor adhesion obtained using prior art alkaline cyanide-free copper and copper alloy strike and plating baths is mainly caused by their tendency to plate a poorly adherent layer of copper or alloying metal onto the zinc substrate by immersion. When the cleaned and properly prepared zinc die-casting is immersed in a strike bath for a strike electrodeposit, there may be a tendency to form a poorly adherent layer of metal from the bath onto the zinc by immersion before electroplating takes place. Such an immersion deposit causes poor adhesion of subsequently applied electrodeposits. The strike bath solution of choice to date for obtaining the required degree of adhesion has been the alkaline cyanide-based copper strike bath which does not plate copper by immersion onto zinc.
The term "coating by immersion" as used herein means that a thin (about 1 to 5 microinches) coating of the metal(s) dissolved in the strike bath plate(s) out on the surface of a metal substrate when the substrate is merely immersed in the bath at the normal bath operating conditions of temperature and bath component concentrations, but in the absence of any electric current applied thereto.
U.S. Pat. No. 4,356,067 discloses the addition of zinc to an alkaline cyanide-free copper plating bath to deposit a copper-zinc alloy. Although the patent generally teaches that its plating baths may have a pH of about 10 to about 13.5, all of the examples in this patent show the use of relatively large amounts of Na, K, or Li hydroxide in the plating bath, which would provide for pHs thereof of about 12 to 14, and ammonium hydroxide is said to be unsuitable since it is not sufficiently alkaline. These baths that are relatively highly alkaline, i.e., those having a pH of above 11.5, tend to readily attack the zinc die-casting placed therein and deposit zinc thereon by immersion which would result in poor adhesion of any coatings made therewith and any subsequent electrodeposits.
Furthermore, it has been found that the glucoheptonate complexing agent used in these baths is not, in fact, a good complexing agent for zinc at any pH, and in addition does cause problems with respect to the waste treating of rinse waters emanating from the use of baths containing such complexing agent since it will not readily permit the residual copper content of the rinse water to be low enough to meet federal and/or local government standards for environmentally discharged waste water. Ideally, the complexing agents used should be readily removable from the waste rinse water together with the metals complexed therewith so that the metal content in these waters is low enough to comply with the minimums set by the relevant government regulations.
Prior to the present invention, therefore, alkaline cyanide-free copper strike baths were not available for the purposes of providing acceptable levels of adhesion between strike coatings applied from such baths and the metal substrates to which such strike coatings were applied, and particularly with rrespect to strike baths whose effluent can readily be treated for the removal of residual metal values.